1. Field of the Invention
This invention concerns labels made from polyolefin films, and a method of making such labels.
2. Description of Related Art
Polyolefin films are increasingly being used to produce labels, both as a substitute for paper labels and to exploit the inherent properties of polyolefin films such as their printability and their ability to be molded into and to adhere to the surfaces of containers as they are being formed. Since polyolefins used to form such films are usually inherently clear and colorless, films formed from them are often rendered opaque and/or colored white, thereby producing a label which is itself opaque.
A variety of techniques have been proposed for making polyolefin films opaque, one being the inclusion of a pigment, for example, titanium dioxide, and another being the formation of microvoids within the films which scatter light and thereby impart opacity to the films. The use of titanium dioxide has the advantage that it generally provides a high degree of opacity, but this is accompanied by its additional cost, which is a factor because significant amounts of titanium dioxide are required in order to obtain the desired opacity. Microvoids have the advantage that they can be formed relatively easily in polypropylene, but the degree of opacity which can be achieved is often insufficient for labels. In addition, the amount of voiding agent which can be added to polypropylene, for example, is limited by the reduction in strength of the resulting voided film. Combinations of a pigment such as titanium dioxide and a voiding agent such as chalk in polypropylene films have therefore been used in an attempt to improve the opacity of labels made from polypropylene.
In certain end uses of labels made from polyolefin films, for example, in in-mold labelling, the film is pre-cut and converted into individual labels which are then formed into a stack from which individual labels are removed as they are required. However, the problem with removing individual labels from a stack is that they are often difficult to separate from each other as a result of static electricity between adjacent labels, and this often increases as more labels are removed from the stack.
The build up of static charge is also a problem if polyolefin films are cut and fed at high speed from reels of film, for example in the cases of wrap-around labelling, cut-in-place labelling, or in the case of pre-formed labels on a release web, for example in self-adhesive labelling.
The undesirable effects of static charge build up can be reduced, for example, by adding relatively large amounts of antistatic agents to the films, but doing so can also lead to other undesirable physical effects such as reducing print adhesion to the films.
Accordingly, it is an object of the invention to provide labels formed from polypropylene polymers that have advantageous properties. It is also an object of the invention to provide methods of making and using such labels.
In accordance with these and other objectives, there has been provided a polymeric label formed from a film comprising a layer of propylene polymer resin having microvoids therein, the microvoids having been formed by stretching a web containing the beta-form of polypropylene.
In accordance with the present invention, there is also provided a method of making a label including:
(a) forming a film comprising the beta-form of polypropylene;
(b) stretching the film to form microvoids; and
(c) cutting the film into labels.
In accordance with the present invention, there is also provided a polymeric label formed from a film comprising an opaque layer of propylene polymer that does not include void-initiating particles or opacifying pigments.
Further objects, features, and advantages of the invention will become apparent from the detailed description of preferred embodiments that follows.